The prime purpose of a belt conveyor system is to move material from one point to another. Weighing is a useful function of materials handling or transportation aspects of a belt conveyor system. Weighing bulk materials in volume may require consideration of storage, feeding, weighing, and other functions, such as readout of weighing data.
Many factors affect design and selection of equipment for weighing bulk materials. For example, material flow characteristics are considered when arranging a weighing system. Particle size, percent moisture, aeration, and free-flowing and non-free-flowing characteristics are a few flow characteristics typically taken into consideration. An automatic weighing system may control the proportioning of bulk materials, such as would be needed for weighing of ingredients into a process that may blend ingredients and may be responsible for weighing during receiving or shipping of bulk materials.
For weighing, the belt conveyor system may incorporate one or more belt scales. Various types of feeders have been developed to feed materials in a belt conveyor system such that a uniformly consistent stream of material may be applied to the belt scale. Since a belt scale senses material over a known length of belting, weight is frequently expressed in terms of weight values per unit of length, such as pounds per foot (lb/ft). Weigh values per unit length are often multiplied by velocity terms, such as feet/minute (ft/min), in order to establish flow rate, such as pounds per minute (lb/min) or tons per hour (TPH).
Belt scales fit into all types of bulk material handling facilities, require little space, and, if designed correctly, may cause no interruption in the flow of material. Belt scales are the most common type of continuous weighing devices, and include one or more conveyor idlers mounted on a structural frame, such as a weighbridge. Medium to high volume weighing typically requires a weigh hopper or continuous weigher. Continuous weigh feeders constitute a special application of belt scales.
Weigh belt feeders generally utilize a belt to draw the material through a shear gate and modulate a speed of the belt to control a flow rate of material being drawn from a storage unit, such as a bin, bunker, silo, hopper, or other suitable storage-type feeder. The material may be fed over a weighing device such as the belt scale. Weigh feeders include the weighing device to control the flow rate of discharge of material from the storage unit. Between the weigh feeder inlet and outlet, the weight values per unit of length of the material may be measured. At any convenient location on the feeder the velocity of the belt may be measured. The product of the two measurements is the flow rate which is compared to a setpoint. If a difference exists between the two, then a signal may be relayed to the feed section so as to increase or decrease the volume entering the feeder. Thus, the weight sensing function is an integral part of the system.
Belt scales are offered with varying numbers of weighing idlers (e.g., rollers equipped with ball bearings over which the conveyor belt passes). Typically, the number of weigh idlers ranges from one to six, but there are cases where the number may exceed six. The weigh idlers are typically spaced apart evenly throughout the weighing region of the conveyor, with a typical spacing being between 2.5 and 6 feet. Belt scales offer a structural weigh span based on the number of weighing idlers. The sensitivity to belt loading at each of the weigh idlers may be the same, as is the case with a “full floating” weighbridge, or it may vary, as is the case with a pivoted weighbridge. A belt load may be transmitted from the weighbridge directly or via a lever system to a load sensing device.
The signal from the load sensing device is usually combined with another signal representing belt speed. The combined output from load and belt speed sensors may provide the flow rate of the material passing over the scale by providing a flow rate (e.g., lbs/min) signal. The belt conveyor system may include a totalizer to integrate the flow rate signal with time, and the total tonnage (e.g., lbs) carried over the belt scale may then be registered and displayed. By weighing material “en route,” a belt conveyor system may quantify flow of material, thus providing a means to monitor the flow pattern in measurable specific terms.
In a belt conveyor system, long weigh spans are favored for weighing accuracy, or totalization of weight, in that they carry heavier loads and provide a greater degree of belt load averaging. As a result, a long weigh span provides greater immunity to extraneous conveyor tension, stiffness influence factors, belting construction irregularities, and sporadic disturbances, such as those that may arise from mechanical shocks and gusts of wind. However, the increased averaging ability of a long weigh span works against a conflicting objective of rapid control response. On the other hand, shorter weigh spans have limited load averaging ability and, thus, are responsive to shorter term fluctuations along the length of the belt. Thus, the length of the weigh span may play an important role in establishing weighing accuracy and control response limits of a belt conveyor system.